The invention relates to a high temperature superconductor (=HTS) coated conductor,
comprising an HTS layer deposited epitaxially on a substrate,
wherein the HTS layer exhibits a lattice with a specific crystal axis being oriented perpendicular to the substrate plane,
in particular wherein the HTS layer material is of ReBCO type and the c-axis is oriented perpendicular to the substrate plane,
wherein the HTS layer comprises particle inclusions,
in particular wherein the particle inclusions may be used to introduce pinning of magnetic flux.
Such a method is known from US 2006/0025310 A1.
Superconductors are used to transport electrical current at low resistance, and thus at low power losses. Superconductors require cryogenic cooling in order to ensure the superconducting state. High temperature superconductor (=HTS) materials have a relatively high critical temperature Tc, as compared to conventional superconductors, and therefore are less expensive in cooling. In particular, some HTS materials allow the use of liquid nitrogen for cooling. On the other hand, the HTS materials exhibit rather high critical current densities at low temperatures of 3 to 20K. This performance is important for high field magnets used in e.g. NMR where fields are so high that the current performance of low temperature superconductors (typically used in this area) becomes insufficient.
Known HTS materials are ceramic, and may be (and typically are) prepared as thin films on a substrate for practical applications (“coated conductor type”). Generally, in order to achieve a high current carrying capacity, the thin HTS films should be practically single crystalline, with a defect density as small as possible. It is known to use buffer layers between a substrate and a HTS thin film in order to provide the crystalline structure of the thin HTS film, or to improve the crystal quality of the thin HTS film.
However, in the regime of use of HTS materials, the HTS materials act as type II superconductors, meaning that magnetic flux may penetrate into a HTS layer, typically as isolated “flux pipes” (in German “Flussschläuche”). The density and position of the flux pipes depends on the magnetic field present (which may be an external field and/or a field originating from the electrical current running through the HTS layer, “self field”). During use of the HTS coated conductor, the magnetic field typically varies, and the flux pipes redistribute accordingly. This redistribution or movement of the flux pipes causes electrical losses, and may decrease the critical current and the critical magnetic field of the HIS coated conductor at which the HTS material quenches (i.e. becomes normally conducting).
It is known that “pinning centers” may reduce losses and increase the critical current and the critical magnetic field in a type II superconductor. The pinning centers make the flux pipes stay at the pinning centers when the magnetic field varies.
US 2006/0025310 A1 discloses an YBCO film with inclusions of BaZrO3 nanoparticles. The film was prepared by pulsed laser deposition (=PLD) with a target of YBCO with 5 mole % of BaZrO3, and resulted in critical currents Ic improved by a factor of about 1.5 to 2 as compared to pure YBCO films under the same conditions.
US 2008/0176749 A1 and US 2009/0088325 A1 describe YBCO films including vertical columns of nanodots of BaZrO3, deposited by PLD with a single target mixture of YBCO and a nanopowder of BaZrO3. An improvement of the critical current Ic of a factor of 6 was observed.
Although the BaZrO3 particles or nanodots may serve as pinning centers, a co-deposition of YBCO (or another ReBCO) material and an additional inclusion material such as BaZrO3 in the desired way is difficult, and requires specifically prepared targets.
It is the object of present invention to provide a more simple method for producing a HTS coated conductor with reduced losses, and with improved critical current and critical magnetic field.